1. Field of the Invention
The present invention generally relates to data transfer methods and systems for use in information processing systems, and particularly relates to a data transfer method and system for use in an information processing system which employs a plurality of different types of memory devices.
2. Description of the Related Art
Together with a recent increase in the sophistication of service functions provided in the field of mobile communications such as cellular phones, the size of data being handled increases, resulting in a need for a larger memory volume. In mobile phones, for example, service functions are provided for moving picture data (movie functions), still images (camera functions), game functions, WEB access functions, etc. The required size of memory has significantly increased compared to the time when voice communication was the main mode of communication.
In mobile telephone equipment, SRAMs (Static Random Access Memory) were used as work memory in the days when voice communication was predominant. An SRAM is comprised of 6 transistors per memory cell, and, thus, the number of devices for storing 1-bit data is relatively large compared to a DRAM (Dynamic Random Access Memory), for example, resulting in difficulties in achieving a large memory capacity. Accordingly, it is extremely difficult to attain such highly sophisticated services as described above by use of a system employing only SRAMs.
Against this background, pseudo SRAMs (PSRAMs) have been developed that use a DRAM as a memory core but provide an SRAM-type interface. The pseudo SRAM can replace the SRAM while taking advantage of the accumulation of past technologies and assets, thereby making it possible to achieve highly sophisticated services by providing a memory system having a large storage capacity.
Many of the memory systems for mobile phones use three types of memories, i.e., an SRAM, a pseudo SRAM, and a flash memory. The flash memory is used to store programs and data. The SRAM is used to store data of relatively small size such as cache data while the power is on. The pseudo SRAM is used as work memory that is used at the time of moving picture processing or the like. Each of these memories is optimized according to its usage, and its replacement is regarded as a difficult challenge. The SRAM is characterized by its low standby current consumption and high-speed read/write access, but is not suitable for achieving a large memory capacity. The SRAM is also volatile. Accordingly, the SRAM is suited to store cache data of small size, but is not suited to store programs and data, which are required to be maintained after the power is off. The flash memory has low read/write access speed, but is characterized by its nonvolatile nature and large capacity as well as low standby current consumption. Accordingly, the flash memory is indispensable for use as storage for storing programs and data which need to be sustained after the power is off. The pseudo SRAM needs refresh operations, and its standby current consumption is large. The pseudo SRAM is also volatile. However, the pseudo SRAM has a large memory capacity and high read/write access speed. Accordingly, the pseudo SRAM is suitable for use as a work memory.
The use of a plurality of memory devices in a single system gives rise to a problem of a cost increase, which results from an increase in the number of memory components inside the mobile telephone equipment. Further, the technology for grinding chips for the purpose of sealing these memories in a single package needs to be developed, and the technology for achieving an MCP (multi-chip package) needs to be developed. These are the issues that need to be addressed.
In order to obviate the problems described above, the number of types of memories used in a system needs to be reduced. In the following, problems that arise when one of the three types of memory devices is replaced with one of the remaining two memory devices will be examined.                1) Flash Memory->SRAM: insufficient capacity, data loss upon power-off        2) Flash Memory->pseudo SRAM: data loss upon power-off, large current consumption        3) SRAM->Flash Memory: lengthy write time        4) SRAM->pseudo SRAM: large current consumption        5) pseudo SRAM->Flash Memory: lengthy write time        6) pseudo SRAM->SRAM: insufficient capacity        
It is difficult to overcome the problem of insufficient capacity with respect to 1) and 6). The option 2) is not viable since data is lost upon power-off. These three options are thus not possible in consideration of the mobile phone specifications and systems of the present day. It thus turns out that the flash memory is indispensable. In consideration of the issue of a write time with respect to 5), it can be said that the pseudo SRAM is also an indispensable memory device. Accordingly, the issue at hand is whether the SRAM can be replaced with another memory.
With respect to 4), the pseudo SRAM has a problem of large current consumption although some pseudo SRAMs have been developed that achieve a standby current compatible to that of the SRAM by use of such functions as a power-down mode and a partial refresh mode. Accordingly, it is preferable to replace the SRAM with the flash memory as in 3) than replacing the SRAM with the pseudo SRAM as in 4). In this case, however, there is a need to solve the problem of a lengthy write time.
Accordingly, there is a need for a data transfer method and system that prevents the length of a time required for writing to the flash memory from appearing on the surface as a system operation when the flash memory is used in place of the SRAM.